Today, after completion of the decoding of the human genome, the correlation between genetic information and disease has been revealed. Under such circumstances, gene therapy for treating disease on a genetic level using a certain gene as a medicine is greatly expected as a method for treating various types of diseases, the treatment of which has been difficult so far.
In order to realize such gene therapy, it is essential to develop a vector that stably transfers a gene to a target site. The characteristics required for such a vector are: (1) a high stability in blood; (2) avoidance from uptake into reticuloendothelial system and renal clearance; (3) selective uptake into a target cell; (4) a smooth transfer into cytoplasm; and (5) low toxicity. That is to say, it is necessary for a gene vector to have two conflicting characteristics, namely, to stably encapsulate a gene therein until uptake into a target cell and to smoothly release the gene after the uptake into the cell (cytoplasm). A virus with excellent infectious ability satisfies such characteristics necessary for a gene vector, but a viral vector is accompanied with a problem regarding a risk of side effects. Thus, currently, attention has been focused on a non-viral gene vector.
As such a non-viral gene vector, a polyion complex (PIC) micelle, which is formed by electrostatic interaction between DNA acting as a polyanion and block copolymers composed of polyethylene glycol (PEG) and polycation (a cationic polypeptide), has been reported to date (please see S. Katayose et al., Bioconjugate Chem., 8, 702-707 (1997); S. Fukushima et al., J. Am. Chem. Soc., 127, 2810-2811 (2005)).
This PIC has a structure wherein the DNA is condensed as a result of the interaction of the DNA with the polycation portion in the block copolymer to form a core portion, and wherein the hydrophilic and bio-compatible PEG portion in the block copolymer forms a shell surrounding the core portion. Thus, the PIC is able to stably encapsulate DNA even in blood, for example. Moreover, since the PIC has a particle size of approximately 100 nm, which is almost the same level as that of virus, it is able to avoid the foreign substance-recognizing mechanism existing in vivo. Furthermore, since an ethylenediamine unit (—(CH2)2—NH—(CH2)2—NH2) contained in the side chain of the polycation in the block copolymer has two pKa values, the PIC also has an advantage whereby endosomal escape is promoted by the proton sponge effect in a cell, while a complex with DNA is formed outside the cell. The aforementioned characteristics of the PIC enabled the improvement of gene expression efficiency.